Researchers to explore differential speed limits

Thursday, June 06, 2013

The Michigan Department of Transportation has awarded a Wayne State University professor with a one-year, $175,000 grant to study differential speed limits on highways.
In Michigan, Indiana and other states, differential speed limits have long been thought to reduce crashes. Trucks need a lower speed limit than cars, officials reasoned, because it takes them longer to stop due to their size. But the presence of differential speed limits has its detractors, and the Michigan DOT hopes that this new study will put the debate to rest once and for all.
Peter Savolainen, an associate professor of civil engineering at Wayne State, will collect historical and new data from Michigan and around the country; review previous research; and survey trucking industry groups during the course of his study. His findings will be presented to the Michigan DOT, which will then enact policy based on the results. Savolainen will be assisted by Wayne State’s Timothy Gates.
The two researchers will focus on safety data, but will also consider effects on pavement and air quality, delays and the potential impact on freight operations.
"Changing the truck speed limits is not expected to impact all types of crashes equally. Consequently, we'll analyze subsets, such as fatal crashes that occur during non-rush hour periods," Savolainen said. "In many cases, speed is not necessarily a problem during rush hour due to high congestion. Similarly, certain types of crashes are more likely to be affected by speed limit policy, such as rear-end collisions.”
Savolainen’s research will ultimately help the DOT and Michigan lawmakers decide between three speed-limit scenarios. Michigan’s current policy, which establishes a speed limit for trucks that is 10 miles per hour lower than that for cars, can be upheld, or the speed difference could be lowered to 5 miles per hour. The third option is to create a uniform speed limit.
Though the debate over truck speed has a long history, Savolainen is confident his research will present a full picture of the issue.
"Previous studies in other states have yielded conflicting results," he said in a statement. "Our study will consider a broader spectrum of factors than prior research. We'll also be taking a more careful look at the metrics we're using to assess safety." - Jon Ross